Comparator

[Maurizio Cimadamore]

I ran your test and I realized that there's still one piece of work left 
to do in this area - which is to use inference constraints during the 
most specific check. The compiler is doing all the right things, but 
then most specific test is currently requiring that all lambda-shaped 
arguments are applied to functional descriptors with the same parameter 
types; here you need a bit of inference to prove that, otherwise the 
parameter types are just different.

Maurizio

On 15/07/13 17:38, Maurizio Cimadamore wrote:
> On 15/07/13 17:28, Marc Petit-Huguenin wrote:
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>> On 07/15/2013 09:18 AM, Maurizio Cimadamore wrote:
>>> On 15/07/13 17:15, Marc Petit-Huguenin wrote: On 07/15/2013 09:09 AM,
>>> Maurizio Cimadamore wrote:
>>>>>> On 15/07/13 16:53, Marc Petit-Huguenin wrote: Hi,
>>>>>>
>>>>>> I am trying to use a comparator on a Map.Entry<String, Integer>
>>>>>> stream, and I am not sure why this does not compile:
>>>>>>
>>>>>> .sorted(Comparator.comparing(Map.Entry::getValue))
>>>>>>> This is as expected - the method reference Map.Entry is defined on
>>>>>>> a generic class with unknown parameter types (some Map<K, V>) -
>>>>>>> therefore the compiler needs the info from the target (in a
>>>>>>> top-down way) in order to decide what the method reference looks
>>>>>>> like. But the target type info is incomplete as it depends on
>>>>>>> inference variables on its own. So the compiler ends up applying a
>>>>>>> simpler (arity-based only) check during overload resolution - which
>>>>>>> means the compiler can't really pick the right comparing() method
>>>>>>> ahead of the target-type. A shorter way to get this to compile
>>>>>>> should be this: .sorted(Comparator.comparing(Map.Entry<String,
>>>>>>> Integer>::getValue))
>>> I forgot to tell that I tried this too.  It fails with a "error: reference
>>> to comparing is ambiguous", but not with the new "inference loop" error.
>>>
>>> Note that ".sorted(Comparator.comparing((Map.Entry<String, Integer> entry)
>>> -> entry.getValue()))" (i.e. without the .intValue()) fails with a similar
>>> error.
>>>> Are you using the official JDK 8 or the experimental lambda-repo
>>>> compiler? Only the latter contains a fix for this.
>> I built it yesterday from the lambda repository:
>>
>>    .:5faccb5b660e corba:08e436473edb hotspot:7b772f9e0365 jaxp:ae467ce203a8
>> jaxws:f2e56f100ecf jdk:00b08a855e2e langtools:f75d9c517541 nashorn:dd371db92a91
> Ok - I'll dig it more and come back to you
>
> Thanks
> Maurizio
>
>
>>>> Maurizio
>>>>>>> Let me know if that works for you. Maurizio
>>>>>> error: reference to comparing is ambiguous error: incompatible
>>>>>> types: Cannot instantiate inference variables T because of an
>>>>>> inference loop
>>>>>>
>>>>>> (interestingly the second error was not present when using a build
>>>>>> from last week)
>>>>>>
>>>>>> These two compile:
>>>>>>
>>>>>> .sorted(Comparator.comparing((ToIntFunction<Map.Entry<String,
>>>>>> Integer>>)Map.Entry::getValue))
>>>>>>
>>>>>> .sorted(Comparator.comparing((Map.Entry<String, Integer> entry) ->
>>>>>> entry.getValue().intValue()))
>>>>>>
>>> -- Marc Petit-Huguenin Email: marc at petit-huguenin.org Blog:
>>> http://blog.marc.petit-huguenin.org Profile:
>>> http://www.linkedin.com/in/petithug
>>>
>> - -- 
>> Marc Petit-Huguenin
>> Email: marc at petit-huguenin.org
>> Blog: http://blog.marc.petit-huguenin.org
>> Profile: http://www.linkedin.com/in/petithug
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